This section is intended to provide a background or context to the invention that is recited in the claims. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and claims in this application and is not admitted to be prior art by inclusion in this section.
The following abbreviations that may be found in the specification and/or the drawing figures are defined as follows:
ACaccess categoryAIFSarbitration interframe spaceAPaccess pointCSMA/CAcollision sense multiple access/collision avoidanceCWcontention windowDIFSdistributed (coordination function) interframe spaceEDCAenhanced distributed channel accessIEEEInstitute of Electrical and Electronics EngineersMPmesh pointPIFSpoint (coordination function) interframe spaceQBSSQoS basic service setQoSquality of serviceSTAstationUEuser equipmentWLANwireless local area network
Contention based networks have multiple users seeking radio resources for uplink transmissions. WLAN is a particular contention based network which users, termed stations or STAs, listen to see if the radio channel is clear and if it is they wait a minimum time period (DIFS) and if the channel is still clear they can transmit. If the channel is busy the STA then backs off a random number of slots and if the channel is not busy after counting down that number of slots the STA can transmit. Then there are other protocols the STA follows if the packet transmission fails. This is termed CSMA/CA in IEEE 802.11e, which is an improvement to prior versions of 802.11 WLAN. CSMA/CA is therefore intended to avoid collisions between simultaneous transmissions from different STAs each contending for a slot in which to transmit.
CSMA/CA based principles perform well particularly when the traffic load is small and there are enough radio resources for the offered load transmission. But studies show that this network channel access technique is less than optimal in the case where STAs are competing on the same resources, and/or if hidden STAs exist.
In some environments that are many access points (APs) located close to one another with possible overlapping bandwidths. Of course network planners seek to avoid situations where multiple APs operate at the same frequency, but in practice it is difficult to achieve in all instances. In many cases the same bandwidth may be in use by two or more APs which also have overlapping physical coverage areas, while simultaneously there is a nearby AP which provides coverage for its cell or area using another frequency and so does not need to share the bandwidth with any of the other APs. Also the WLAN APs are very often applied in homes and end users may locate the WLAN APs close to each other and using the same frequency, and they may not be capable or volunteer to spend time and effort to configure the APs appropriately.
There is a problem of traffic load balancing, which obviously becomes more important when the network traffic load increases. Under high load conditions the data transmissions require more time since more often the STA's contention based access attempts will fail to find a free channel. This of course degrades the power save performance from the STA's perspective, since it has to spend more time listening to the channel to find a free transmission opportunity.
When the media sharing concept was developed it was considered that it should provide efficient and fair opportunities for all STAs to transmit and receive data, and for this reason it is not a priority-only contention system. STAs with lower priority data still compete for radio resources with the STAs having higher priority data and can sometimes prevail which satisfies the fairness concern. A mechanism assures that the STAs with higher priority data obtain a transmission opportunity at a statistically higher rate than the STA with lower priority data which satisfies the efficiency concern. But where the traffic load is high, the contending STAs are still competing for the same fixed pool of radio resources made available by the AP.
The latest improvements to WLAN are that the contention based channel access mechanisms (EDCA in 802.11e) provide QoS support and AP prioritization for infrastructure networking. QoS is managed by EDCA parameters which help prevent the higher priority traffic from being interfered by lower priority traffic. For example, email data might be a lower priority than voice over WLAN traffic. The CSMA/CA approach also divides traffic into different access categories (ACs), each having traffic of different priorities. In practice, under IEEE 802.11e the APs apply their own set of EDCA parameters that typically allow more prioritized channel access (for example, calculation of shorter backoff values) than the EDCA parameters of the terminals/STAs. IEEE 802.11e also describes WLAN controlled channel access which is not contention-based, but at least the contention based access (with the EDCA parameters) has no way to balance the traffic load among cells.